Regenerating spent cooking liquors produced in making paper pulp



Oct. 7, 1958 A. M. THOMSEN 2,855,272.

. REGENERATING SPENT cooxmc LIQUORS PRODUCED IN I MAKING: PAPER PULP Filed Nov. 22, 1955 FHA-6Z0; l/dzatdes caalerr I Mmpau- #1440121 I n Kaolin; m, {'ystem v MENTOR.

United States Patent REGENERATIN G SPENT COOKING LIQUORS PRODUCED IN MAKING PAPER PULP 7 Alfred M. Thomson, San Francisco, Calif.

Application November 22,1955, Serial No. 548,525

1 Claim. (Cl. 23-63) This application is a continuation, in part, of a disclosure previously made and bearing Ser. No. 165,885, filed June 3, 1950, now abandoned, being itself a continuation-in-part of U. S. No. 2,510,668. The instant application is concerned solely with that phase of said disclosure relating to the regeneration of the resident alkali metal salts in such a manner that they pass through the temporary phaseof alkali metal cyanides before becoming once more employed as a cooking liquor.

I will commence my description by taking a special type of such a liquor, that produced in Kraft cooking, though my process has a much wider application. Subsequently I shall describe sundry modifications that makes it available for practically any and all types of pulp making. Inasmuch as wood cooked ,by this well known method is generally pulped with 25% of its weight in a mixture of caustic soda and sodium sulphide and rather more than half of said wood, by weight, is put into solution, it follows that said liquor will consist roughly of one-third soda salts as originally added and two-thirds of organics derived from the wood. Variations in wood species and individual preferences prevent a more accurate specification as to actual composition of said liquor after its use in the cooking operation.

The aim and object of my process is to recover for subsequent re-use the soda salts present in said liquor and simultaneously achieve a better utilization of the carbon resident in the contained organics, in part, in the form of hydrogen cyanide, same constituting a valuable byproduct.

I wish to emphasize at this point that the chemistry involved in such recovery of soda salts and such production of hydrogen cyanide is very old, but though the chemical reactions are old I believe the application of same in the manner herein combined and executed is definitely new and will constitute a valuable addition to the pulp industry.

In my specific illustration which is illustrated in the drawing I take, as previously stated, spent liquor from the kraft process, evaporate it in conventional manner and then dessicate it completely raising the temperature employed to over 500 F. at the close soas to carbonize the organics. In this manner I obtain a very porous and yet coherent product consisting of alkali metal salts and carbon, the latter constituting about 40% of the total weight. In order to increase this percentage of carbon 1 commingle with the evaporated liquor before carbonization a carbonaceous residue obtained at a later step in my process and I can thus raise the content of carbon to virtually any extent desired. The advantage of this step will be commented upon at a later point in my description.

I then subject the carbonaceous or carbonized product to the action of highly heated nitrogen gas which is absorbed by the soda salts with the formation of cyanides,

2 regenerating a spent cooking liquor from the pulping operation.

The cyanized mass from the nitrogen treatment step is next cooled and leached with water yielding a solution of alkali metal cyanide and a residue consisting largely of carbon. As before mentioned this residue is in part commingled with fresh evaporated liquor prior to carbonizat on. Inasmuch as the ratio of carbon to soda in the liquor residue, without such admixture of re-cycled carbon, is greater than is actually consumed in the process, such re-cycling can theoretically raise the carbon ratio to any extent desired. The aim and object of such increase of carbon is to present a dry mass entirely without stickiness in the nitrogen absorption phase. I believe that much if not all of the difiiculty experienced in the past by other operators is inherent in any operation which does not use a carbon-alkali metal complex so constituted that the fused salts during nitrogen absorption remain securely held in a carbon matrix while still permitting the free passage of nitrogen between individual lumps, and through diffusion, access to the interior of said lumps. This object is attained in the manner herein described to better advantage than I have been able to find anywhere in the appropriate literature which is very extensive.

The solution of sodium cyanide obtained in the leaching step is next commingled with carbon dioxide. Hydrogen cyanide is evolved and is recovered by conventional means, such as compression to the liquid state in which form it is shipped. The solution now consists largely of carbonate of the alkali metal used in cooking the pulp, with shome sulphocyanide which may be ignored or, if it be desired to save it, recovered. If the solution be saturated with carbonate it is only necessary to cool same to obtain a copious crystallization of carbonate which is readily separated by thecentrifuge leaving a mother liquor correspondingly high in sulphocyanide.

It will be obvious that having thus obtained the major part of the resident alkali metal of the spent cooking liquor in the form of carbonate the recovery is finished as it may be re-cycled to any conventional cooking step; thus: for kraft, sulphate, or soda cooking it will be recycled just prior to the causticizing step; for sulphite work it will be treated with sulphur dioxide. It is thus seen that my process delivers a reclaimed soda that can be used by any process, and conversely, any liquor can be used in place of kraft, the only difierence residing in the amount of sulphocyanide yielded as this chemical cannot be regarded as having any place in conventional pulping. However, inasmuch as cyanide in any form is far more valuable than any alkali metal carbonate such conversion is not a fault from an economic view point.

The expression any liquor may require clarification.

1 The conventional lime based acid sulphite cook liquor side of this disclosure.

cannot be used, per se. But if treated with sodium sulphate and/or carbonate and the lime salts precipitated removed, then it will function as indicated. Likewise, the liquor resulting from the magnesium based acid sulphite can be treated with sodium carbonate, as yielded by my process, and is then acceptable. If it be thought that the sulphur removal from the kraft liquor is objectionable it is, of course, corrected by increasing the sulphide content from another furnace to compensate.

The method selected to make the carbon-alkali metal complex is in itself of no moment as I consider it as out- A convenient method, however, is to feed a slurry of the mixed recycled carbonaceous residue and evaporated liquor to a, rotary kiln heated by direct fire to such a temperature that finished material, having the general appearance of cement clinker, issues from the firing end.

I have already called attention to the importance of having the mass to be cyanized in the correct mechanical condition of a most intimate mixture of soda salts and carbon while retaining it in the form of actual lumps that still are extremely porous. Another very important feature of my process is the way this mass and thenltrogen ,by which itlis ;trav,ersed.is heated to. reaction tempera ture herewithqspecified -as anythnig between .-.a low of 1700: 1'1. ,a1;d.ahigh of 2600 F. Below the lower temperature the reaction isso slow'as to-become impractical andabove the;higher lirnit,'volatilization of the cyanide and formation .oflcyanamidebothzcontribute to neutralize the ,good result, ;i. e., .speed of reaction, which naturally iSqfllCtIBSj-llli of the elevated temperature. Heatingzis accomplished inrthefollowing manner.

Thermassbeingcyanided is contained in a chamber so placed between, twoheataccumulators that nitrogen traversing said mass alternately .in opposite direction is heated by; abstracting. heat fromone of said regenerators while conveying heat to the-other regenerator after having passed through the reacting mass. At a oint intermediate between the reaction chamber and the heat accumulator absorbing heatsufficient air is admitted to burn virtuallyall the carbon monoxide made in said reaction. Thegases entering theheating or. absorbing accumulator are thus increased'in temperature by the heat developed by said burning carbon monoxide and this heat is likewise stored in said accumulator thus furnishing all the heat required. The reaction chamber and its contained mass is not heated save by heat transferred from the traversing nitrogen which becomes the heat carrier of the reaction, this being strongly endothermic. The correction of previous attempts of making cyanide by the joint elfect of the two improvements herein disclosed virtually removes=all formerly encountered difficulties.

To accomplish its work as a heat carrier the nitrogen must bepassed through the apparatus at a rapid rate, and this in turn is favorable to the reaction which is impeded by .the presence of more thansmall-amounts of carbon monoxide. It is obvious that the speed of flow will govern the amount of such carbon monoxide which should not be allowed to exceed 7% of-the introgen as it issues from the reaction chamber. Control is established by continuous gas readings on the carbon dioxide in the cool .gas issuing from the absorbing heat accumulator. If the current of nitrogen be so regulated as to keep the carbon dioxide below 10% excellent results will be certain to follow.

The source of the nitrogen is found in the process itself. The addition of air to burn the carbon monoxide produced inthe reaction will furnish more nitrogen than the process'consumes .so a part must be voided. All that is required is tostrip the cooled gas issuing from the absorbing-accumulator of its increment of carbon dioxide by any conventional means :such as absorption in water under pressure, in solutionsof alkaline carbonates,,in caustic washes, or in organic absorbing media. Further instructions would be superfluous.

It is likewise obvious that the time elapsing between each reversal of flow through the entire system is a matter concerned not with the process but with its apparatus. If the heat accumulators be relatively large then considerable time, say 10 minutes may elapse between each reversal, if they be made smaller as a matter of decreasing the capital cost then a more rapid reversal is mandatory. In any event the temperature of a definite spot should not vary more than F. from the start to the finish of a uni-directional flow. If the variation be greaterthan that then the reversal time must be shortened to avoid damage to the basic accumulators.

Throughout this disclosure the terms soda salts and alkali metal salts have been used indiscriminately. In practice, of course, all commercial use is with the cheaper soda, nevertheless as potash could be substituted advantageously, due to more ready nitrogen absorption. I have used the term alkali metal as a generic expression for either potassium or sodium or to a mixture of both.

Having thus fully described my process with a preferred version as applied to a kraft cooking liquor, I claim:

The method of recovering the alkali metal salts contained in a spent cooking liquor derived from the making of paper pulp, in a form suitable .for re-cycling into said cooking operation, which comprises; evaporating said liquor and commingling same with a carbon residual derived from a later step; carbonizing saidmixture to yield an intimate mixture of alkali metal salts and carbon in the form of a coherent, porous body; traversingsaid carbonized substance with nitrogen gas at-a temperature between 1700 F. and 2600 F. until the constituent alkali metal salts are substantially converted into the corresponding cyanides; leaching the cyanized mass'with water to produce a solution of alkali metal cyanides .anda residue of carbon; re-cycling said carbon residual to'where previously specified; commingling said cyanide solution with carbon dioxide thus obtaining a solution of alkali metal carbonate suitable for re-cycling into thepulping operation, with attendant evolution of hygroden cyanide gas.

References Cited in the file of this patent UNITED STATES PATENTS 576,264 Gilmour Feb. ,2, 1897 1,308,184 McAfee Iulyl, 1919 1,408,754 McElroy Mar; 7, 1922 2,275,792 Moseby Mar. 10, 1942 2,495,248 Gagliarde Jan. 24, 1950 2,552,183 Knight May 8, 1951 2,586,030 Groombridge -Feb. 19, 1952 

